This proposed work is designed to test the hypothesis that the depression in food intake of animals fed a diet deficient in one of the key nutrients (e.g., essential amino acids) is mediated by certain neural sensitive areas ("receptor") responding to metabolite levels in the central nervous system and possibly elsewhere. We intend to examine the dietary patterns and choices of animals ingesting disproportionate amounts of amino acids using recording balances for detailed monitoring. Such measurements will alss apply in rats subjected to the following treatment (a) norma: (b) lesions in certain areas of the brain: (c) infusion with amino acids (possibly including radioactive isotopes) drugs and neural humors via cannulas implanted in the brain; the carotid artery or the portal or jugular vein: (d) electrical stimulation in certain areas of the brain via implanted electrodes. Also, key metabolites in the blood plasma, brain and other tissues will be analyzed to correlate their levels with their dietary pattern. Areas of the brain to be initially examined include the hypothalamus, prepyriform cortex, amygdala, hippocampus, septum and the midbrain. The results from these studies should help elucidate the location and something about the nature of the postulated neural sensitive area.